1. Field of the Invention
The present invention relates to an optical disk drive, and in particular, to a tracking servo control method and device for preventing a tremble of an object lens during a fast search mode.
2. Description of the Related Art
Compact disks (CD), a kind of an optical disk, are typically used for recording audio data such as music. Recently, however, the CDs are widely used as an auxiliary storage for text and computer data because of its high storage capacity.
In general, the CDs have a large number of tracks arranged at fine intervals of 1.6 .mu.m, wherein the number of the tracks is about twenty thousand for recording music of one hour. Therefore, in a microscopic view, the CDs may rotate acentrically. From the standpoint of an optical pickup, each track with a width of about 0.5 .mu.m rotates (or proceeds) sinusoidally. A tracking servo allows the optical pickup to accurately search (or seek) the tracks and radiate a laser beam to them. A fundamental structure of the tracking servo is illustrated in FIG. 1.
In FIG. 1, reference numeral 2 denotes an optical disk (i.e., CD), reference numeral 4 denotes a disk motor, and reference numeral 6 denotes a pickup. As illustrated, the pickup 6 includes an object lens 30, a tracking coil 32, a magnet 34, a 1/4-wavelength plate 36, a beam splitter 38, an optical detector 40 and a laser diode 42. Further, reference numeral 8 denotes a feeding motor for feeding the pickup 6, reference numeral 10 denotes a tracking error detector 10, reference numeral 12 denotes a phase compensator, reference numeral 14 denotes a system controller, and reference numerals 16 and 22 denote adders. Moreover, reference numerals 18 and 24 denote drivers, and reference numeral 20 denotes a lowpass filter (LPF).
In operation, the tracking error detector 10 generates a tracking error signal according to a beam trace status, and the phase compensator 12 generates a phase compensation signal according to the tracking error signal. The tracking servo corrects the position of a laser beam spot by moving the object lens 30 and the pickup body 6 in any direction according to the phase compensation signal. After tracking, the pickup 6 follows a specific track. For moving the pickup 6, the adder 22 adds a search control signal output from the system controller 14 to an output signal of the lowpass filter 20 which lowpass-filters the phase compensation signal output from the phase compensator 12. The driver 24 then drives the feeding motor 8 according to an output signal of the adder 22, to move the pickup 6 in any direction. Further, for moving the object lens 30, the adder 16 adds the search control signal output from the system controller 14 to the phase compensation signal output from the phase compensator 12, and the driver 18 drives an actuator (comprising the tracking coil 32 and the magnet 34) according to an output signal of the adder 16, to move the object lens 30.
FIG. 2 illustrates a schematic block diagram of a conventional tracking servo control device for preventing a tremble of the object lens during a fast search (or course seek) mode. Referring to FIG. 2, a switch 54 has a common node connected to the input of the adder 16, a contact node A connected to an output of the phase compensator 12 and a contact node B connected to an output of an amplifier 52. In the conventional tracking servo control device, the pickup 6 outputs a central-point servo signal for positioning the object lens 30 at a stable central point, to prevent a tremble of the object lens 30 during the fast search. The central-point servo signal is amplified in the amplifier 52 and then applied to the contact node B of the switch 54. The phase compensator 12 outputs the phase compensation signal according to the tracking error signal from the tracking error detector 10 and provides the phase compensation signal to the contact node A of the switch 54.
FIG. 3 illustrates a diagram for explaining the tracking error signal and the central-point servo signal, in a heterodyne tracking servo. In FIG. 3, reference numeral 60 denotes a photodiode divided into A, B, C and D areas, and reference numeral 62 denotes an optical spot. The tracking error signal output from the tracking error detector 10 has a value of (A+C)-(B+D), and the central-point servo signal output from the pickup 6 has a value of (A+D)-(B+C). As can be appreciated, the central-point servo signal and the tracking error signal have opposite phases relative to each other.
FIG. 4A illustrates a frequency and a servo-on time of the tracking error signal in the state where the object lens 30 does not tremble during slow search (or fine seek), and FIG. 4B illustrates a frequency and a servo-on time of the tracking error signal in the state where the object lens 30 trembles during the fast search (or course seek). As illustrated, the servo-on time during the fast search of FIG. 4B is delayed by a time t as compared with the servo-on time during the slow search of FIG. 4A. Accordingly, track searching is delayed by the time t.
To prevent the delay in searching the tracks, the system controller 14 switches the common node of the switch 54 to the contact node B slightly before the servo-on time. The adder 16 then adds the search control signal to the central-point servo signal output from the amplifier 52 instead of the phase compensation signal output from the phase compensator 12. As a result, the driver 18 provides the tracking coil 32 with a driving current corresponding to the central-point servo signal. The central-point servo signal having the phase opposite to the phase of the tracking error signal, stabilizes the object lens 30 which was trembling by a driving current corresponding to a tracking signal.
However, the conventional tracking servo control device is disadvantageous in that it cannot use a pickup which does not output the central-point servo signal. Further, although the pickup outputs the central-point servo signal, the central-point servo signal, the central-point servo signal itself may have a deviation, which makes it difficult to accurately stabilize the trembling object lens.